feat: add feature carousel hero + enable Stripe billing

- Replace static dashboard screenshot with interactive feature carousel
  9 slides: Dashboard, Mitglieder, Kurse, Finanzen, Veranstaltungen,
  Newsletter, Website, Buchungen, Dokumente
  Auto-advances every 6s, clickable sidebar + bottom tabs
  Virtual app UI rendered with shadcn components (no images needed)

- Enable Stripe test mode billing
  Add publishable key to .env.development, .env.production, docker-compose
  Add secret key to .env.development and docker-compose
  Add NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY to Dockerfile build args

Dockerfile

@@ -21,6 +21,7 @@ ARG NEXT_PUBLIC_DEFAULT_LOCALE=de
 ARG NEXT_PUBLIC_ENABLE_FISCHEREI=true
 ARG NEXT_PUBLIC_ENABLE_MEETING_PROTOCOLS=true
 ARG NEXT_PUBLIC_ENABLE_VERBANDSVERWALTUNG=true
+ARG NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=
 ENV NEXT_PUBLIC_CI=${NEXT_PUBLIC_CI}
 ENV NEXT_PUBLIC_SITE_URL=${NEXT_PUBLIC_SITE_URL}
 ENV NEXT_PUBLIC_SUPABASE_URL=${NEXT_PUBLIC_SUPABASE_URL}
@@ -29,6 +30,7 @@ ENV NEXT_PUBLIC_DEFAULT_LOCALE=${NEXT_PUBLIC_DEFAULT_LOCALE}
 ENV NEXT_PUBLIC_ENABLE_FISCHEREI=${NEXT_PUBLIC_ENABLE_FISCHEREI}
 ENV NEXT_PUBLIC_ENABLE_MEETING_PROTOCOLS=${NEXT_PUBLIC_ENABLE_MEETING_PROTOCOLS}
 ENV NEXT_PUBLIC_ENABLE_VERBANDSVERWALTUNG=${NEXT_PUBLIC_ENABLE_VERBANDSVERWALTUNG}
+ENV NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=${NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY}
 RUN pnpm --filter web build
 
 # --- Run ---